A conventional drilling rig comprises a drill string composed of drill pipes and drill collars and having at its lowermost end a drill bit provided with a bit cone used to cut the rock formation into which a bore hole is being drilled and also fluid ejection means to erode the rock formation as the drill bit is lowered and to wash the cuttings away. The fluid ejection means generally comprises one or more nozzles in the drill bit through which hydraulic fluid such as drilling mud is pumped under pressure from a mud pump located on the surface (i.e., above ground) of the drilling rig. After the drilling mud has been expelled through the nozzle orifices it is conducted through an annular region concentric with the drill string back to the surface of the drill rig wherein it is separated from particulate matter such as shale and repumped back through the drill string.
Given the redoubled efforts which have been taking place recently in an effort to locate new oil deposits due to the ever increasing cost of petroleum and petroleum products, it has become necessary to drill new wells to greater and greater depths through all manner of rock formations. In working at these greater depths it is, as well, important to be able to control the direction taken by the drilling apparatus. That is, it is often necessary to deviate the bore hole at an angle from a straight vertical path to get to a particular deposit. This technique is particularly important in off-shore drilling where it is desirable to place as few production platforms as possible while yet having the ability to drill throughout and produce from as wide an area as possible.
Deep wells pose serious obstacles, however, which limit the effectiveness of the erosion capability of conventional drilling systems due to the opposing hydrostatic pressure of the drilling mud once it has been ejected through the drill bit nozzle orifice. Hydrostatic pressure here refers to the fact that drilling mud, once it has left the drill string, exerts what is in effect a static back pressure against the drilling mud which is being ejected from the drill bit. This downhole pressure is essentially equal to .rho.H where .rho. is the density of the drilling mud and H is the height of the annular mud column, equal to the vertical depth of the bore hole. Thus it can be seen that hydrostatic pressure which jetting drilling mud must counteract linearly increases with increasing vertical depth of the bore hole.
Thus in the art of oil drilling strenuous attempts have been made to discover new means whereby the velocity of hydraulic drilling fluid jetted through drill bit nozzles can be increased such that the erosional and directional capabilities of the drilling system as a whole can be correspondingly increased. For example, in my prior U.S. Pat. No. 3,599,733, herein incorporated by reference, I have described a method for enhanced directional jet drilling utilizing a reciprocatable in-hole mud pump located relatively adjacent to a directional jetting bit in the drill string, which bit has at least one fluid outlet orifice through which a relatively greater amount of drilling mud can be caused to flow than through other bit orifices. In essence, this inhole mud pump comprises an extensible cylinder and piston section having a back pressure operated fluid valve at the top thereof. In the technique described in my prior patent, this extensible section is first fully extended to fill the cylinder with mud after which the drill string is lowered, thus trapping the cylinder full of mud behind the back pressure valve resulting in increased compressional forces to force the mud out of the jetting bit at an increased velocity. By having one relatively larger nozzle in the bit disposed relatively adjacent the vertical bore hole wall, the bore may be deviated in a direction corresponding to the position of that nozzle.
Similarly, in my prior U.S. Pat. No. 3,815,692, also herein incorporated by reference, I apply the same general principles as in U.S. Pat. No. 3,599,733, but therein the drill string is disclosed to be constructed to include an extensible hydraulic cylinder and piston section located above a drill bit positioned at the bottom of the drill string and wherein the drilling system may include any kind of conventional jetting and/or other drilling bit. The extensible section is positioned in a partially retracted or less than fully extended position such that substantially the whole force urging the drill bit against the bottom of the bore hole comprises the hydraulic force generated within the extensible hydraulic cylinder and piston section when pressurized fluid is admitted into that section and trapped therein by means of a check valve located at the top thereof. Whatever force is applied to the trapped pressurized fluid by slacking off on the surface vertical drive means controlling the amount of drill string weight allowed to urge the bit against the bore hole bottom is transmitted to the bit. The pressure may be maintained by compressing the extensible section for the distance of available travel, all the while jetting drilling mud at increased velocity over that obtainable in the absence of said extensible portion to erode the hole with increased efficiency.
The reciprocatable down-hole mud pump (of the type sold by the R. F. Varley Co., La., under the trademark "HJD") described above from U.S. Pat. No. 3,815,692 is hereinafter sometimes also referred to as the "down-hole pump". Viewed alternatively from the description given above, when the down-hole pump is reciprocated it works like a pump, filling its top part with drilling mud as it is opened and ejecting the mud from one or more bit nozzles as it is closed (i.e., telescoped or compressed inwardly on itself). The ejection pressure is a direct function of the speed with which the pump is closed, higher pressures of course yielding more effective mud jets.
The jetting bit itself usually has one large nozzle (e.g., 3/4 inch), the jetting nozzle, and two smaller nozzles (e.g., 1/4 inch) which are blanked out if jetting is to be done at depths exceeding 5,000 feet.
In spite of advancements such as the ones described above for increasing the velocity of jetted drilling fluid, new and/or improved means for even further enhancing hydraulic jetting velocities are constantly being sought and, to the extent that they are discovered, improved drilling systems, straight ahead or directional, can be developed which will permit drilling to deeper and deeper depths. Such a technique is the subject of the present invention.